1. Field of the Invention
The invention in general relates to sensors for measuring distance to metallic objects, and particularly to a rugged sensor of the eddy-current variety.
2. Description of the Prior Art
Eddy-current sensors are well known and operate on the principle that the impedance of an ac-excited electrical coil is subject to change as the coil is brought in close proximity to a metallic object. For a given coil location, geometry and excitation frequency, the coil's inductance and resistance change is a function of the electrical conductivity, magnetic permeability, and geometry of the metallic object.
Instruments utilizing the eddy-current sensor are well known for measuring displacement or vibration in various types of machinery. By way of example these sensors are utilized to monitor the blading arrangement in a steam turbine as described in co-pending application, Ser. No. 413,638 filed Aug. 31, 1982, now U.S. Pat. No. 4,518,917, and assigned to the same assignee as the present invention. Commercially available sensors utilized in the hostile turbine environment are subject to failure due to various causes such as vibration induced cracking of the sensor housing, erosion of sensor parts due to abrasive damage from water droplets which may impact at a velocity as high as 1,200 feet per second, and electrical wire breakage due to rough handling during installation.
The sensor of the present invention obviates the deficiencies of the prior art sensors so as to allow its use in environments where other sensors have failed.